The invention relates to a device for the pyrometric measurement of the temperature of a graphite tube furnace for flameless atom absorption spectroscopy, having a radiation receptor for generating a temperature-dependent signal and an amplifier which follows the radiation receptor.
Devices for pyrometric temperature measurement of this type are generally known. The particular problems of applying them in flameless atom absorption spectroscopy consist in the fact that it is desirable to be able to cover with a single device the entire temperature range through which the graphite tube furnace passes through its heating up process. This temperature range extends from about 300.degree. K. to about 3,300.degree. K.
According to Wien's law of displacement, the maximum of the spectral intensity distribution of a black body shifts towards the short-wave spectrum with increasing temperature. In the temperature range mentioned, the maximum shifts from about 10 .mu.m to about 1 .mu.m. During this process, the total radiation increases proportionally to T.sup.4. If the total radiation is used, a pyrometer intended for measuring this temperature range would supply an output signal which changes over several orders of magnitude within the measuring range. The dynamic ranges of the known radiation detectors and of the amplifiers following them is not nearly adequate for this purpose. For this reason, it is necessary to limit the change of the radiation flux, which is connected with the temperature change, at the radiation receptor in a suitable manner so that the output signal changes with the temperature within tolerable limits.
In Analytical Chemistry, Volume 46 (1974), pages 1028-31, a measuring arrangement is described which covers a temperature range of between 550.degree. C. and 2,600.degree. C. Restriction of the radiation flux is achieved by a suitable choice of the sensitivity of the radiation receptor. A photodiode is used, the sensitivity of which is between 0.62 .mu.m and 2 .mu.m. The spectral range below 0.62 .mu.m is additionally cut off by a red filter. This combination represents a measuring window which is located on the short-wave slope of the spectral intensity distribution curves of the radiator and which, at the same time, determines the lower range of the temperature measurement. This is because the spectral intensity distribution curves which are valid for temperatures lower than 550.degree. C. do not supply any further measurable contribution for wavelengths below 2 .mu.m.
The same principle of selecting a suitable measuring window is also utilized by the measuring device described in German Patent Specification No. 2,627,254. In this arrangement, the selection was made in dependence on the lowest temperature to be measured. This was supposed to be 100.degree. C. For this reason, the measuring window was placed on the long-wave slopes of the intensity distribution curves in the spectral range from 8 .mu.m to 14 .mu.m. An interference filter is provided for limiting the spectral range. All other optical and optoelectronic elements of the arrangement were selected in such a manner that they do not have any wavelength-dependence within the measuring window. A thermocouple-type detector was taken as radiation receptor and a rock salt or germanium lens was used for concentrating the radiation flux on the detector. The lenses can also be taken as carriers for applying the interference filter. The upper measurable temperature is about 2,700.degree. C.